using phong_ramp as a factor?
Your problem is that phong_ramp doesn't actually internally calculate that factor; as a BSDF, it's a closure, which is a function to be called by the renderer, later, with more data available. So, while it may appear on the surface that it's internally calculating those colors, it's only setting up the material to provide an environment for their calculation during the actual ray-trace render. (OSL is completely ignorant of lighting.)

So, as much as I hate answering questions with a stout negative, no, you can't determine that factor from OSL. (It literally doesn't even exist yet.) However, if you're looking for a reflectance factor, it's fairly easy to calculate on its own. You do have the surface normal N, the incident ray from the viewer I, and while the light data may not be available you can always substitute in a parameter point, or even hard coded point, representing the anticipated light.

The Blinn-Phong algorithm, for all its brilliance, isn't that complicated to understand and is effectively cited in full on Wikipedia. I'm not sure what you're hoping to do with that factor but I wish you luck in doing it and hope this helps! Another thought might be to interpolate eight equidistant points between black and white for the shader, and simply multiply it by a color or other closure color to tune it, but it ultimately depends on what you're aiming for.